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Assumptions Used in Estimating Fair Value of Stock Options Granted (Detail) - 2002 Stock Incentive Plan
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Expected term 5 years 9 months 18 days 6 years 3 months 18 days
Volatility 79.43% 86.30%
Risk-free interest rate 1.98% 1.42%